Radio frequency identification (RFID) is a technology that has been in use since the 1940's where military aircraft carried large transponders as part of an IFF (Identify Friend-or-Foe) system. The transponder received electrical power from the aircraft and was thus an active RFID transponder. When a radar signal interrogated the transponder the transponder would generate a specific radio frequency signal that identified the aircraft as a “friendly” aircraft. This IFF system prevented otherwise friendly aircraft from being shot down by other friendly aircraft or friendly military forces.
State of the art microelectronics technology now make it possible to fabricate very small analog (e.g. RF circuitry) and digital (e.g. Memory) circuits on silicon (Si). As a result, RFID technology is currently being used to obtain information stored on a RFID tag that is a much smaller version of the aforementioned large RFID transponder used for aviation IFF. At its most basic, a RFID system includes a RFID reader and one or more RFID tags that are attached to an object to be identified.
The RFID reader transmits a radio frequency signal that creates an electromagnetic field. The RFID tags include electronics that store information about the object the tag is attached to. For example, the object can be a piece of merchandise, a food article, currency, a product, a component passing through a manufacturing process, an automobile, or a piece of luggage. The RFID tag also includes an antenna and electronics connected with the antenna for receiving a specific radio signal and for transmitting the stored information at a specific radio frequency when the RFID tag enters the electromagnetic field generated by the RFID reader.
A RFID tag can be an active tag or a passive tag. An active RFID tag includes a power source, such as a battery, for example. Upon entering the electromagnetic field generated by the RFID reader, the active RFID tag extracts data from the electromagnetic field and then transmits its own information carrying radio signal to the RFID reader. In contrast, a passive RFID tag does not include a power source. Instead, the electromagnetic field generated by the RFID reader induces an AC voltage in the antenna of the passive RFID tag and that induced voltage is then rectified to produce a DC voltage that energizes the passive RFID tag. Once energized, the passive RFID tag transmits an information carrying radio signal to the RFID reader. Due to the requirement of a power source, active RFID tags are typically larger and more costly than passive RFID tags.
In FIG. 1, a substrate 400 includes a plurality of RFID chips 401 that include an area a1. The substrate 400 can be a wafer of a semiconductor material such as silicon (Si), for example. The substrate 400 can include a wafer flat 400f and scribe lines 402s that delineate the RFID chips 401 and allow the RFID chips 401 to be separated from one another.
A designer of an RFID chip 401 is faced with two fundamental choices between using an on-chip antenna 405 as depicted in FIG. 2a or an external antenna (421, 431) as depicted in FIGS. 2b, 3a, and 3b. In FIG. 1, The RFID chip 401 can include the on-chip antenna 405 positioned within an outer perimeter p1 of the chip 401, RFID electronics 403 that occupy a smaller area a2, and conductive traces or bonding wires 413 that electrically connect nodes (415, 417) on the RFID electronics with nodes (407, 409) on the on-chip antenna 405.
If the on-chip antenna 405 can be accommodated on-chip without increasing the area a1 of the RFID chip 401, then the RFID chip 401 will offer the lowest possible RFID tag cost because tag cost is directly proportional to the area a1. However, one disadvantage of the on-chip antenna 405 is that unless the chip 401 is large, the on-chip antenna 405 will offer only a very limited range. That is, the chip 401 must be in very close proximity to the RFID reader in order to receive the electromagnetic field and to transmit the information stored on the RFID chip 401 to the RFID reader. The range may be adequate in some cases, but in general more range is better. Another disadvantage of on-chip antennas is that scaling of the RFID chip 401 to smaller chip sizes (i.e. reducing the area a1 thereby reducing tag cost) is largely precluded because shrinking the on-chip antenna 405 will seriously impact the range of the RFID chip 401 and/or reduce a data rate at which the information is transmitted from the RFID chip 401 to the RFID reader.
On the other hand, by using an external antenna as depicted in FIGS. 2b, 3a, and 3b, the range of the RFID chip 401 can be greatly increased, but at a substantial increase in tag cost. The increase in tag cost can be attributed in large part to: a cost of manufacturing the external antenna (421, 431); a cost of mounting the RFID chip 401 to a substrate 451 that carries the antenna 431; and a cost of making an electrical connection (between the RFID chip 401 and the external antenna (421, 431). For example, in FIGS. 2b, 3a, and 3b, a wire bonding process can be used to connect a wire 413 with nodes (415, 417) on the RFID chip 401 and with nodes (423, 425) on the external antenna (421, 431). Solder balls 444 or other techniques that are well understood in the microelectronics art (e.g. surface mount technology) can be used to electrically connect the RFID chip 401 with the external antenna (421, 431).
The process of connecting the RFID chip with the external antenna is a non-trivial process that increases the cost of the RFID tag, especially when the RFID chip 401 is much smaller than the external antenna (421, 431) as is often the case when large external antennas are used. For example, the μ-chip™ by HITACHI® has a size that is 0.4 mm*0.4 mm, which is much smaller than a grain of rice; however, the external antenna that is connected with the μ-chip™ is substantially larger than the μ-chip™ itself. Much effort has been expended in recent years to develop a low-cost means for connecting a small RFID chip to a large external antenna. As one example, Alien Technology® claims a RFID tag cost of less than $0.10 in high volumes for RFID chips that are connected with a large external antenna using fluidic self assembly (FSA) techniques. HITACHI® with its μ-chip™ and other makers of RFID tags have developed their own approaches to solving the problem of connecting a small RFID chip to a large external antenna. As another example, a current cost per unit area for a RFID chip fabricated on silicon (Si) is on the order of $0.20/sq-mm and with a RFID chip size of 0.4 mm on a side, the cost for the bare RFID chip (i.e. absent the external antenna) would be roughly $0.03 per RFID chip (i.e. $0.20/mm2*[0.4 mm*0.4 mm]=$0.03 per RFID chip). Therefore, the total cost of a complete RFID tag would then be determined by the cost of the large external antenna and the cost of connecting the antenna to the RFID chip.
Consequently, there exists a need for a RFID tag with a cost that approaches that of an on-chip antenna, but with a performance approaching that of a separately fabricated and much more expensive external antenna. There is also a need for a low cost method of fabricating a RFID tag with a low cost antenna that uses a low cost means for connecting the antenna with a RFID chip and would add little to a cost of even the smallest RFID chips.